With increasing development of technology, computers become essentials of our lives. As common electrical appliances, reliable and stable power is necessary for activating the computers. As known, a power supply apparatus is widely employed to convert an alternating current (AC) from a regular plug into a direct current (DC) to be used by the computer. For a purpose of maintaining desirable performance of the computer, the power supply apparatus should meet with specified requirements and specifications associated with safety, reliability, protection, EMC (electromagnetic compatibility), etc.
Referring to FIG. 1(a), a functional block diagram of a conventional power supply apparatus is shown. The power supply apparatus comprises an AC-to-DC converter 11 and a DC-to-DC converter 12. An input AC voltage Vin received by the AC-to-DC converter 11 is firstly converted into a high DC voltage V, which is then converted by the DC-to-DC converter 12 into a low DC voltage Vout. The low DC voltage Vout is outputted to be used by a load 13 such as an electrical appliance.
FIG. 1(b) is a schematic circuit diagram of the AC-to-DC converter in FIG. 1(a). The AC-to-DC converter 11 comprises a first diode D1, a second diode D2, a bridge rectifier 111, a filter 112, a first resistor R, a second resistor r and a voltage detecting circuit 113.
During normal operation of this power supply apparatus, the input AC voltage Vin should be maintained within a predetermined range, for example from 90 to 130 Volts. The voltage detecting circuit 113 is employed to dynamically detect whether the input AC voltage Vin is acceptable. When the detected AC voltage Vin lies in the acceptable range, the power supply apparatus continuously operates. Whereas, if the detected AC voltage Vin lies outside the acceptable range, a control signal indicative of an insufficient voltage is asserted from the voltage detecting circuit 113. In response to the control signal, the power supply apparatus stops operation.
By means of the bridge rectifier 111 and the filter 112 of the AC-to-DC converter 11, the input AC voltage Vin is converted into the high DC voltage V. When each of the voltages inputted into the diodes D1 and D2 is larger than that of the voltage Vc across the capacitor C, the diodes D1 and D2 are conducted, so that the input AC voltage Vin, as shown in FIG. 1(c), is rectified by the diodes D1 and D2 to produce a DC voltage signal, as shown in FIG. 1(d). Subsequently, the capacitor C is charged via the first resistor R until each of the voltages inputted into the diodes D1 and D2 is less than that of the voltage Vc across the capacitor C. Then, the capacitor C is discharged via the second resistor r until each of the voltages inputted into the diodes D1 and D2 is larger than that of the voltage Vc across the capacitor C. The charging/discharging procedures are continuously performed.
The voltage detecting circuit 113 comprises a comparator 1131 and a voltage source 1132 with a reference voltage Vref, and is employed to dynamically receive the DC voltage Vc of the capacitor. The received DC voltage Vc is compared with the reference voltage Vref of the voltage source 1132 by the comparator 1131. According to the comparing result, a control signal is generated to control the operation of the power supply apparatus.
For a purpose of avoiding immediately asserting the control signal to repeatedly turn on/off the power supply apparatus if the comparing result does not meet the requirement, the comparator 1131 should be rendered to have hysteresis. That is to say, the control signal is not asserted from the comparator 1131 immediately when the comparing result does not meet the requirement, but is asserted when the voltage Vc of the capacitor C is less than the hysteresis voltage Vhy (as shown in FIG. 1(e)) so as to turn off the power supply apparatus.
As known from the circuit of FIG. 1(b), the capacitor C is discharged via the resistor r. Since the RC circuit is discharged in an exponential manner, a higher value of the resistance of the resistor r multiplied by the capacitance of the capacitor C results in a slower discharging rate. As shown in FIG. 1(e), the response time Tfail1 of the RC circuit from the moment when the comparing result does not meet the requirement to the moment when the voltage Vc across the capacitor C is less than the hysteresis voltage Vhy is considerably long. Such a long response time leads to an inferior performance of the power supply apparatus.
Therefore, it is needed to provide a voltage detecting circuit that can solve the drawbacks in the prior art.